This invention relates to geo-location systems, which determine the unknown location of an object or node by employing signal time-of-flight information from known transmitting locations to determine the distance between the known locations and the unknown location of the receiving node or object.
There are two general types of geo-location systems, one type employing narrow band electromagnetic waves and the other ultra-wide band waves. An example of a narrow band geo-location system is the Global Positioning System (GPS), which has a carrier frequency of 1575.42 and a chip rate of 1.023 MHz (0.06% fractional bandwidth). An example, of an ultra wide-band system is the precision locations system made by MultiSpectral, Inc., that uses a 400 MHz wide signal at 1400 MHz carrier frequency (27% fractional bandwidth).
Geo-location systems rely on the time of flight from known fixed location beacons. The receiving node or object whose location is to be determined needs to receive at least three beacons from known location transmitters in order to fix its location in space. However, these systems rely on the positioning being transmitted directly from the transmitting beacons, whereas in fact a signal from a transmitting location may be both received directly but also received via reflection from an interposed object.
This problem can be seen in FIG. 1 which is a generic depiction of a geo-location system including three transmitters 10, 11, and 12 which transmit beams 15, 16, and 17 towards the node 20 whose location is to be determined. Beams 16 and 17 go directly to receiver 20 but beam 15 is split with beam 15a being reflected off a reflecting object 22, such as a metallic object, so that its time of flight is not a true indication of the distance between transmitter 10 and receiver 20. Further, beam 15b may have to pass through an absorbing obstruction 23, such as foliage, which does not cause the beam to be reflected but considerably reduces its power. Thus if the receiver 20 is to distinguish between the two beams 15a and 15b on the basis of which is the stronger of the two, it would select beam 15a, thereby providing erroneous information to the determination of the location of the receiver based on the three received beams.
Another problem with present geo-location systems is that GPS systems cannot be used to determine the location of nodes or objects within buildings or behind walls, as the short wavelength signal of such systems does not penetrate through walls. Ultra-wide band geo-location systems have the ability to penetrate walls and other non-reflecting objects due to the low frequency content of their signal. However, ultra-wide band systems occupy such large bandwidths that, even through there power is low, they can interfere with other currently operating systems. Ultra-wide band time-of-flight systems will also suffer the same ambiguity problem arising for reflections and absorbing obstructions, as discussed above.
It is therefore an object of our invention to provide geo-location systems, which are not subject to the problems and defects of these known systems, as discussed above.
In accordance with one aspect of our invention the receiving node discriminates between directly received signals and those whose path included a reflection on the basis of the polarization of the received signal. When an electromagnetic wave is reflected from a surface, its polarization is generally shifted, such as from right hand circular to left hand-circular, which is referred to as depolarization. The received de-polarized signal is considered cross polarized to the original signal. Since the transmitter is always transmitting the same polarization, which is known to the receiver, any received signals having a polarization different from the polarization as transmitted, is a cross polarized signal which indicates a reflection having occurred in the path. While a path, which includes multiple reflections, could cause the receiver to receive a signal with the same polarization as the original signal, such a received signal would be at a much lower power level and would not significantly interfere with the determination of the receiver""s location.
In accordance with another aspect of our invention wherein the transmitted signals are ultra-wide band signals, in addition to distinguishing between multi-path signals and direct signals based on signal polarization, transmission occurs in discontinuous frequency bands to provide the benefit of low frequency penetration and high frequency resolution without creating interference over a large frequency range with existing systems.